1. Field of the Invention
The present invention relates to a heat exchanger which has thermal stress relieving zones. The invention is for use particularly, though not exclusively, in a radiator for a motor vehicle.
2. Discussion of the Related Art
Typical vehicle heat exchangers, such as automobile radiators, include a plurality of thin walled tubes disposed between a pair of headers. The ends of the tubes are rigidly connected to the headers, and fluid can pass from one header to the other via the tubes. The tubes are interleaved with corrugated fins, and the tubes and fins are supported by a pair of side supports which extend between the headers and are rigidly secured thereto. The component parts of the heat exchanger are first assembled and then connected together by brazing or welding according to the materials from which each component is constructed.
A function of each side support is to limit deformation of the tubes close to the edge of the heat exchanger occurring as a result of internal pressure in the heat exchanger. The fins between the side support and the tube nearest to the side support are important for the transfer of loads between the tubes and the side support.
It is important to provide means for relieving thermally induced stress in the side support. This stress arises when coolant heats up the tubes, causing them to expand more rapidly than the side supports. If the side supports and the tubes are made from different materials, having different coefficients of thermal expansion, the stress build-up would be exacerbated. Longitudinal stress induced by this differential expansion can result in premature failure of the heat exchanger.
In a known design of heat exchanger, a thermal stress relieving zone comprises a linear saw cut made across each side support, which severs the side support completely through. A problem with saw cutting is that it is very noisy, difficult to automate, and produces a lot of metal fines resulting in increased downtime and maintenance of the saw.
It is-known to make a cut using a lancing technique, which requires the use of a side support which has a generally planar base portion and a pair of flanges extending generally longitudinally to the axes of the side supports. A linear slot is formed in the base by a stamping operation prior to securing the side support between the headers and to the fins. After the side support has been secured to the headers and the fins, the flanges are then sheared by a lancing technique at points adjacent the slot. The lancing completely fractures the flanges, forming the thermal stress-relieving zone as a gap which completely separates one end of the side support from the other end.
The lancing technique requires a relatively wide slot in order to provide an adequate target for the lance cutter. This technique reduces the problems associated with saw cutting, but with the resulting wide gap, the fin support necessary to transfer loads between the side support and the tube is locally lost, resulting in reduced fatigue life under repetitive pressure cycling.
It has been proposed in U.S. Pat. Nos. 5,165,153 and 5,257,454 to make a heat exchanger that includes a side support as described above having a gap disposed at an angle with respect to a plane perpendicular to the longitudinal axes of the side supports. These patents, which are incorporated herein by reference, also disclose an apparatus and a method for making the heat exchanger.
Such angular gaps in the side supports allow each adjacent fin to be supported by the side support on at least a part of its surface. However, they have a drawback of producing asymmetric stress patterns which may result in torsional moments on the side support, thus reducing the lifetime of the heat exchanger. Moreover, the manufacturing process is made more complicated because it is necessary to ensure that each radiator is correctly orientated during the cutting stage so that the parts of the flanges adjacent the slot are correctly positioned by the cutters. It is an object of the present invention to reduce at least one of the above mentioned problems.